Re: [PATCH v3 2/5] vhost-user.rst: Clarify enabling/disabling vrings

[Stefan Hajnoczi]

On Fri, Sep 15, 2023 at 12:25:27PM +0200, Hanna Czenczek wrote:
> Currently, the vhost-user documentation says that rings are to be
> initialized in a disabled state when VHOST_USER_F_PROTOCOL_FEATURES is
> negotiated.  However, by the time of feature negotiation, all rings have
> already been initialized, so it is not entirely clear what this means.
> 
> At least the vhost-user-backend Rust crate's implementation interpreted
> it to mean that whenever this feature is negotiated, all rings are to
> put into a disabled state, which means that every SET_FEATURES call
> would disable all rings, effectively halting the device.  This is
> problematic because the VHOST_F_LOG_ALL feature is also set or cleared
> this way, which happens during migration.  Doing so should not halt the
> device.
> 
> Other implementations have interpreted this to mean that the device is
> to be initialized with all rings disabled, and a subsequent SET_FEATURES
> call that does not set VHOST_USER_F_PROTOCOL_FEATURES will enable all of
> them.  Here, SET_FEATURES will never disable any ring.
> 
> This interpretation does not suffer the problem of unintentionally
> halting the device whenever features are set or cleared, so it seems
> better and more reasonable.
> 
> We should clarify this in the documentation.
> 
> Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
> ---
>  docs/interop/vhost-user.rst | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
> index bb4dd0fd60..9b9b802c60 100644
> --- a/docs/interop/vhost-user.rst
> +++ b/docs/interop/vhost-user.rst
> @@ -409,12 +409,20 @@ and stop ring upon receiving 
> ``VHOST_USER_GET_VRING_BASE``.
>  
>  Rings can be enabled or disabled by ``VHOST_USER_SET_VRING_ENABLE``.
>  
> -If ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been negotiated, the
> -ring starts directly in the enabled state.
> -
> -If ``VHOST_USER_F_PROTOCOL_FEATURES`` has been negotiated, the ring is
> -initialized in a disabled state and is enabled by
> -``VHOST_USER_SET_VRING_ENABLE`` with parameter 1.
> +If ``VHOST_USER_SET_FEATURES`` does not negotiate
> +``VHOST_USER_F_PROTOCOL_FEATURES``, rings are enabled immediately when
> +started.
> +
> +If ``VHOST_USER_SET_FEATURES`` does negotiate
> +``VHOST_USER_F_PROTOCOL_FEATURES``, each ring will remain in the disabled
> +state until ``VHOST_USER_SET_VRING_ENABLE`` enables it with parameter 1.
> +
> +Back-end implementations that support ``VHOST_USER_F_PROTOCOL_FEATURES``
> +should implement this by initializing each ring in a disabled state, and
> +enabling them when ``VHOST_USER_SET_FEATURES`` is used without
> +negotiating ``VHOST_USER_F_PROTOCOL_FEATURES``.  Other than that, rings
> +should only be enabled and disabled through
> +``VHOST_USER_SET_VRING_ENABLE``.

The "Ring states" section starts by saying there are three states:
"stopped", "started but disabled", and "started and enabled". But this
patch talks about a "disabled state". Can you rephrase this patch to use
the exact state names defined earlier in the spec?

>  
>  While processing the rings (whether they are enabled or not), the back-end
>  must support changing some configuration aspects on the fly.
> -- 
> 2.41.0
>

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